In the production of retreaded tires of truck and bus radial tires, a method in which the tread surfaces of tires (hereinafter referred to as tire casing) worn down to reach the end of a primary life are buffed and in which retreaded tread rubber parts (pre-cured treads) vulcanized in advance are stuck thereon has so far been known as one of typical methods. The above method is called in the name of a cold method or a pre-cure method, and the method is distinguished from a hot method in which an unvulcanized tread rubber is put on a tire casing and in which the tire is subjected to mold vulcanization.
In the pre-cure method described above, usually carried out is a method in which an unvulcanized cushion rubber is first stuck on a tire casing in order to adhere a pre-cured tread to the tire casing and in which the pre-cured tread is then stuck thereon and heated at a temperature of 100 to 140° C. in a vulcanization oven.
The cushion rubber used in the above method is provided with a function in which the cushion rubber flows into buffed grain of a tire casing to smoothen the adhesion surfaces and in which the cushion rubber is covulcanized with both of the tire casing and the pre-cured tread to thereby secure an adhesive property between the tire casing and the pre-cured tread.
The above cushion rubber has to be provided with a high cross-linking degree by vulcanization in a relatively low temperature range (close to 100° C.), and the cushion rubber is preferred to have a fast vulcanization speed close to 100° C. in order to enhance vulcanization productivity. On the other hand, in a certain case, the cushion rubber is stuck on the tire casing in a state in which the cushion rubber is rolled in advance into a sheet form, and the cushion rubber is coated directly on the tire casing in another case by means of an extruding equipment. In either of the rolling or extruding steps, the temperature is elevated up to 70 to 100° C., and therefore if the vulcanization proceeds in the rolling or extruding steps, it becomes impossible to covulcanize the interface. Accordingly, the longer scorch time is preferable so that the vulcanization does not proceed for some extent of period at a temperature close to 100° C. That is, having antinomies, high Fmax value of a vulcanization maximum torque which indicates a high cross-linking degree at a temperature close to 100° C. and slow vulcanizing rate at T0.1, at which curing the rubber is initiated (longer time) in contrast to fast vulcanizing rate at T0.9 which is the terminal stage for the vulcanization of the rubber (shorter time) have to be made highly compatible with each other.
Known as a rubber composition suitable for a cushion rubber which is required to provide the above properties are, for example, (1) tires characterized by using a rubber composition prepared by blending a specific thiuram compound, a compound such as dibenzothiazyldisulfide, and a compound such as amines as cushion rubbers (refer to, for example, patent document 1 filed by the present applicant), and (2) a rubber composition prepared by blending a vulcanization accelerator such as benzothiazyldisulfide a vulcanization ultra-accelerator such as tetrabenzylthiuram disulfide, and an amine activator for curing (refer to, for example, patent document 2).
In the art of patent document 1 described above, it is prescribed that at least one compound selected from amines, guanidines, aldehydeamines, and aldehydeammonias is added as the amines and the like described above in an amount of 0.1 to 2.0 parts by weight, however, a superiority of the aldehyde amines among the above amines and the like is not referred therein. In addition, a preferable blending amount range thereof is small, and the blended components are different from those in the present invention.
In the art of patent document 2 described above, a vulcanization accelerator such as benzothiazyldisulfide is used in combination with an amine activator for curing. Relating to the vulcanization ultra-accelerator such as tetrabenzylthiuram disulfide, only the group consisting of tetrabenzylthiuram disulfide and zinc dibenzyldithiocarbamate is prescribed, and other compounds containing no carcinogenic nitrosoamine precursors are neither described nor indicated at all. Thus, a rubber composition in which having antinomies, high cross-linking degree at a temperature close to 100° C. and slow vulcanizing rate at T0.1, at which curing the rubber is initiated (longer time) in contrast to fast vulcanizing rate at T0.9 (shorter time) are made highly compatible with each other and which is excellent in a productivity has not yet been obtained in the present circumstances.
Further, a satisfactory rubber composition has not yet been obtained as well in tires produced by interposing a rubber composition for adhesion between separately vulcanized rubber members and covulcanizing in the present situation.